/* alpha_combine.c - Alpha channel combination, based on Gimp 1.1.24
 * The GIMP -- an image manipulation program
 * Copyright (C) 1995 Spencer Kimball and Peter Mattis
 *
 *  This library is free software; you can redistribute it and/or
 *  modify it under the terms of the GNU Library General Public
 *  License as published by the Free Software Foundation; either
 *  version 2 of the License, or (at your option) any later version.
 *
 *  This library is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 *  Library General Public License for more details.
 *
 *  You should have received a copy of the GNU Library General Public
 *  License along with this library; if not, write to the Free
 *  Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston,
 *  MA 02110-1301, USA.
 */

#include "wraster.h"

void RCombineAlpha(unsigned char *d, unsigned char *s, int s_has_alpha, int width, int height,
                   int dwi, int swi, int opacity)
{
  int x, y;
  int t, sa;
  int alpha;
  float ratio, cratio;

  for (y = 0; y < height; y++) {
    for (x = 0; x < width; x++) {
      sa = s_has_alpha ? *(s + 3) : 255;

      if (opacity != 255) {
        t = sa * opacity + 0x80;
        sa = ((t >> 8) + t) >> 8;
      }

      t = *(d + 3) * (255 - sa) + 0x80;
      alpha = sa + (((t >> 8) + t) >> 8);

      if (sa == 0 || alpha == 0) {
        ratio = 0;
        cratio = 1.0;
      } else if (sa == alpha) {
        ratio = 1.0;
        cratio = 0;
      } else {
        ratio = (float)sa / alpha;
        cratio = 1.0F - ratio;
      }

      *d = (int)*d * cratio + (int)*s * ratio;
      s++;
      d++;
      *d = (int)*d * cratio + (int)*s * ratio;
      s++;
      d++;
      *d = (int)*d * cratio + (int)*s * ratio;
      s++;
      d++;
      *d = alpha;
      d++;

      if (s_has_alpha)
        s++;
    }
    d += dwi;
    s += swi;
  }
}
